1. Field of the Invention
The present invention relates to the fabrication of superconducting wires and rods. More particularly, the present invention primarily relates to the fabrication of superconducting wires and rods made from Yttrium Barium Copper Oxide and Bismuth Strontium Calcium Copper Oxide.
2. Brief Description of the Related Technology
High T.sub.c superconductivity was first discovered in Yttrium Barium Copper Oxide (YBCO) in 1987. Since then, much progress has been made in discovering other compounds and in understanding the fundamental properties of these materials. At the same time, significant effort has been directed towards identifying the potential applications of these new materials, and developing the processing conditions for fabrication of useful forms of these materials, such as wires and rods.
Some of the large scale applications of superconducting wires are in power generation and transmission, high field magnets, magnetic energy storage, and current leads for low T.sub.c superconducting magnets and magnetic energy storage systems. These materials are also used in other applications like electronic devices and magnetometers.
Conventionally, wires of ductile materials are made by extrusion or by rolling and drawing processes. These processes cannot be directly applied to the high T.sub.c superconductors, which are ceramic materials and not capable of plastic deformation. Several variations of these processes are being developed, however, to adapt them to ceramic materials. These include the powder-in-tube process used to fabricate metal-clad composite and the plastic extrusion process used to fabricate bare wires and rods. Other processes like surface coating techniques are also being pursued. See S. Jin, R. C. Sherwood, R. B. Van Dover, T. H. Tiefel and D. W. Johnson, Jr., Appl. Phys. Lett., 51, 203 (1987); M. T. Lanagan, R. B. Poeppel, J. P. Singh, D. I. Dos Santos, J. K. Lumpp, U. Balachandran, J. T. Dusek and K. C. Goretta, J. Less Common Metals, 149, 305 (1989); S. R. Su, M. O'Connor, M. Levinson and P. G. Rossoni, Physica C, 178, 81 (1991); L. D. Woolf, W. A. Raggio, F. E. Elsner, M. V. Fisher, R. B. Stephens, T. L. Figueroa, C. H. Shearer, J. D. Rose, K. M. Schaubel, R. A. Olstad, T. Ohkawa, D. M. Duggan, M. DiMartino and R. L. Fagaly, Appl. Phys. Lett., 58, 534 (1991); and H. Shimizu, H. Kumakura and K Togano, Jpn. J. Appl. Phys., 27, L414 (1988), which are incorporated herein by reference.
The powder-in-tube process uses a preform composed of a nominal ductile metal tube packed with fine superconducting powder. A wire is rolled, drawn, or both rolled and drawn in successive stages until the required final dimensions are achieved. The resulting composite superconducting wire is easily wound into coil shapes. When the radius of the resulting superconducting wire is small, the wire is first wound into a coil and then the coil is heat treated to sinter the powder.
In the case of the plastic extrusion process, superconducting powder is mixed with a set of organics to prepare a paste, which is then extruded to form the desired cross sections. Some of the advantages of this process over the powder-in-tube process are the following: (i) there is no problem of degradation or contamination from other material; (ii) oxygen annealing is comparatively easy, since the surface is exposed; (iii) the monolithic nature of the wire makes it useful for applications like current leads, where metallic cladding is undesirable; and (iv) this process can be applied to the fabrication of different sections and large cross-section area rods.
The plastic extrusion process consists of several stages like paste preparation, extrusion, drying, binder burn-out, sintering and oxygenation. Each one of these stages involves several parameters which affect the quality of the final product and its microstructure. The plastic extrusion process itself is used in the fabrication of certain conventional ceramic shapes. However, in prior attempts at applying the plastic extrusion process to the fabrication of superconducting wires and rods, the additives and processing parameters used have not achieved wires and rods having desired and consistent electrical/superconducting and mechanical properties.
The present invention provides the additives, processing parameters, and processing stages necessary to achieve desired and consistent electrical/superconducting and mechanical properties. For example, the present invention: (i) uses polymeric additives which form an extrudable paste but do not degrade the final properties of superconductor; (ii) optimizes the amount of each of the additives; (iii) optimizes extrusion parameters like compaction load and extrusion rate; and (iv) provides for the removal of solvent first and other polymeric additives later through proper heat treatment and sintering parameters.